1. Field of the Invention
The present invention is directed to a control device for prescribing a time-variable output quantity, particularly for use in setting a time curve of a gradient current supplied to a gradient coil in a magnetic resonance tomography apparatus by a gradient amplifier.
2. Description of the Prior Art
The exact prescription of a time-variable output quantity is required, for example, for setting a gradient current in a gradient coil of a magnetic resonance tomography apparatus. The time curve of the gradient current must be exactly prescribed in accordance with the measuring sequence which is employed for operating such an apparatus. For example, in the control technique disclosed in U.S. Pat. No. 5,349,296, control data sets are essentially pre-calculated before the sequence start. Gradient pulses are defined by their starting amplitude and ending amplitude, and a control computer emits corresponding ramp signals as an output. The rise and decay rates are limited by the performance capability of the gradient amplifiers and by the inductivity of the gradient coils. Given such a prescription of starting and ending values for ramps of gradient pulses, care must always be exercised to insure that the allowable rise and decay rates are not exceeded. This involves increased calculating outlay.
In a number of nuclear magnetic resonance tomography apparatuses commercially offered by Siemens AG, time-variable output quantities for gradient coil systems are calculated by digital signal processors, which are appropriately programmed for this purpose. For registering oblique and double-oblique magnetic resonance tomography images, multiplications with rotational matrices must, among other things, be implemented by the signal processor for calculating the corresponding output quantity. For eliminating calculating time, increments are formed for the output quantity for this purpose, and these increments are subjected to the aforementioned multiplication. Finally, the output quantity is formed by adding a multiplied increment to the momentary value of the output quantity at specific clocked times. In order to keep the influence of rounding errors low, a 16-bit signal processor undertakes the calculations using double word arithmetic of 32 bits. Monitoring of allowable rise and decay rates can be implemented in a simple way with the aforementioned method on the basis of the increments. The aforementioned method for prescribing a time-variable output quantity for the gradient currents, however, occupies calculating power of the digital signal processor that is then not available for other tasks.
In nuclear magnetic resonance tomography, frequency and phase encoding of the resonance signals of the origination of the signals dependent on the location is effected in different directions by gradients, the data encoded in this manner being referred to as the k-space presentation. The following definition applies for the k-space: ##EQU1##
wherein y is thereby the Larmor constant and G.sub.x, G.sub.y, G.sub.z are respective magnetic field gradients in the x, y and z directions of a Cartesian coordinate system. The resolution of the k-space (i.e., the fineness of the incrementation of the data values therein) is critical for the measuring precision. It follows from the aforementioned equations for the k-space that the smallest discrete incrementation is defined by a time grid .DELTA.t and the rastering of the amplitude of the gradients G.
The time grid .DELTA.t is prescribed by the digital control system employed i.e., by its digital signal processors, and typically is 10 .mu.s. Usually, physical quantities are presented with 16 bits by the digital signal processors of the control system. If a maximum amplitude of .+-.25 mT is assumed, approximately 1 .mu.T is obtained as the smallest amplitude unit. The smallest presentable k-space distance between successive values then amounts to approximately 1 .mu.T.multidot.10 .mu.s. This resolution, however, is already too coarse for some applications.